In general, nuclear imaging that is based on detecting gamma rays is well known. In a Compton scatter camera, for example, basic operating principles of which are illustrated schematically in FIG. 1, a source object of interest 10, e.g., a particular organ within the human body, is caused to be impregnated with a radioisotope which emits gamma radiation. A gamma photon travels along a first path of travel 12 until it interacts with an electron of an atom within the detector at location A within a first detector 14. The gamma photon is then deflected or scattered by a Compton scatter angle θ and travels along a second path of travel 16 until it interacts with another electron at location B within a second detector 18.
The scatter angle Θ can be calculated from the amount of energy deposited in each of the detectors 14 and 18 at locations A and B, respectively, from the relation
            E      sc        =                  E                  i          ⁢                                          ⁢          n                            1        +                                            E                              i                ⁢                                                                  ⁢                n                                      ⁡                          (                              1                -                                  cos                  ⁢                                                                          ⁢                  Θ                                            )                                            511            ⁢                                                  ⁢            keV                                ,where Esc is the energy of the scattered photon and Ein is the energy of the incident photon. Thus,
      cos    ⁢                  ⁢    Θ    =      1    -                            511          ⁢                                          ⁢                      keV            ⁡                          (                                                                    E                                          i                      ⁢                                                                                          ⁢                      n                                                                            E                    sc                                                  -                1                            )                                                E                      i            ⁢                                                  ⁢            n                              .      
The actual direction from which the incident gamma ray has come, however, cannot be determined. Therefore, for each detected gamma ray/electron interaction event (consisting of all individual gamma ray/electron interactions associated with a given incident gamma ray), a conic arc 20 exists in the image plane representing the intersection of all possible lines of travel of the incident gamma ray and the image plane that could correspond to the calculated Compton scatter angle.
A more detailed discussion of the basic operating principles of a Compton scatter camera and the disclosure of an improved Compton scatter camera can be found in U.S. Pat. No. 5,175,434, the disclosure of which is incorporated by reference.
In positron emission tomography (PET) imaging, basic operating principles of which are illustrated schematically in FIG. 2, a positron 22 is emitted by the source object 10, which has been caused to be impregnated with a positron-emitting radioisotope. Within a few millimeters from the point of emission, the positron 22 combines with an electron 24 and a pair of gamma rays 26 are emitted as the positron is annihilated. The two gamma rays travel in diametrically opposed directions, which are independent of the original direction of travel of the positron, and strike opposing scintillating crystal detectors 28, 30 through which the gamma rays scatter. Because the gamma rays travel at the speed of light, which is extremely large in relation to the distance between the detectors 28 and 30, the gamma photons strike the detectors 28 and 30 at, for all intents and purposes, exactly the same time.
As the gamma rays scatter throughout the detectors, visible light is emitted from each particular crystal pixel (see FIG. 2A) in which a gamma ray/electron interaction occurs. This visible light is transmitted through glass light pipes 32, 34 to a series of photomultiplier tubes 36, which are used to decode the x- and y-locations of the gamma ray/electron interactions within each of the detectors 28 and 30, i.e., in which individual crystal within the array of crystals comprising the detector the gamma ray/electron interactions occurred.
Because an atom which is struck by a gamma ray will not emit light immediately upon being struck by the gamma ray, and because the gamma rays scatter throughout the detectors at the speed of light such that all scintillating pixels will scintillate at essentially the same time, it is impossible to determine the pixel in each detector in which the first gamma ray/electron interaction occurred. Accordingly, the location at which each gamma ray initially strikes each detector is assumed to be at the centroid of the scintillating pixels.
The point at which the positron was annihilated and the two gamma rays were generated is then assumed to be located along a “line of interaction” extending between the location in each of the two detectors at which the initial gamma ray/electron interaction is assumed to have occurred, and an image of the source object is generated from a multitude (i.e., on the order of millions to tens of millions) of such lines of interaction, as is known in the art. The line of interaction generated for each set of coincident gamma ray/electron interaction events has a certain amount of inherent uncertainty or error associated with it, however, due to the “averaging” associated with assuming that the location of initial gamma ray/electron interaction is at the centroid of the scintillating pixels.